Double shank blind bolt

ABSTRACT

A high strength blind bolt type radially expanding fastening system which may be completely installed from one side of the joint comprising an external 70* flush head sleeve member, and an internal conical pin member having a serrated surface with a break-off groove. The sleeve member is provided with a matching smooth conical inner wall having a taper of approximately 0.020 inch per inch and of correspondingly smaller diameter than that of the pin member. The oversized pin is inserted into the sleeve and the fastener assembly placed into a close tolerance hole from the flush side of the joint. Pulling and holding forces are applied to the pin and sleeve members respectively, and the pin is drawn into the sleeve; a serrated lead-in surface located ahead of the break-off groove acting to react a portion of the load required to expand the sleeve until the serrated lead-in surface and the break-off groove emerge from the flush end of the sleeve. The break-off groove is designed to fracture at a predetermined load sufficient to have expanded the sleeve approximately 0.005 inches thereby causing the serration notches to lockingly engage the smooth inner wall of the sleeve to secure the joint.

United States Patent Schmitt [451 Nov. 7, 1972 [54] DOUBLE SHANK BLINDBOLT [72] Inventor: Hubert A. Schmitt, Auburn, Wash.

[73] Assignee: The Boeing Company, Seattle,

Wash.

[22] Filed: March 31, 1971 [21] Appl. No.: 129,775

[52] U.S. Cl ..85/77 [51] Int. Cl ..Fl6b 10/19 [58] Field of Search..85/77, 78, 72, 70, 4, 82, 83, 85/73, 74; 287/189.36 F

[56] References Cited UNITED STATES PATENTS 2,384,321 9/1945 Lees..85/78 2,538,623 l/l95l Keating ..85/78 3,065,661 11/1962 Kolec et a1..85/72 3,271,058 9/1966 Anderson ..85/4 3,276,308 10/1966 Bergere..85/77 3,286,580 11/1966 Jeal ..85/77 3,369,442 2/1968 Darby et al...85/70 FOREIGN PATENTS OR APPLICATIONS 596,275 12/1947 Great Britain..85/78 602,652 5/1948 Great Britain ..85/77 Primary Examiner-MarionParsons, Jr. Attorney-Glenn Orlob, Kenneth W. Thomas and Bernard A.Donahue [5 7] ABSTRACT A high strength blind bolt type radiallyexpanding fastening system which may be completely installed from oneside of the joint comprising an external 70 flush head sleeve member,and an internal conical pin member having a serrated surface with abreak-off groove. The sleeve member is provided with a matching smoothconical inner wall having a taper of approximately 0.020 inch per inchand of correspondingly smaller diameter than that of the pin member. Theoversized pin is inserted into the sleeve and the fastener assemblyplaced into a close tolerance hole from the flush side of the joint.Pulling and holding forces are applied to the pin and sleeve membersrespectively, and the pin is drawn into the sleeve; a serrated lead-insurface located ahead of the break-off groove acting to react a portionof the load required to expand the sleeve until the serrated lead-insurface and the break-off groove emerge from the flush end of thesleeve. The break-off groove is designed to fracture at a predeterminedload sufficient to have expanded the sleeve approximately 0.005 inchesthereby causing the serration notches to lockingly engage the smoothinner wall of the sleeve to secure the joint.

1 Claim, 4 Drawing Figures DOUBLE SIIANK BLIND BOLT BACKGROUND OF THEINVENTION 1. Field of The Invention This invention relates to a highstrength blind bolt for aircraft structures which is internally lockingand does not require external holding means such as a threaded nut towithstand loads encountered in' an aircraft vibration and fatigueenvironment. The fastener is radially expanding under the action ofmatched conical surfaces, one of which is provided with serrationnotches, to create an interference fit hoop-tension preload of the outersleeve of the fastener and also of the material surrounding the hole forimproved fatigue life.

2. Description of The Prior Art I In the past, many varieties of blindbolts and rivets which may be completely installed from one side of thejoint have-been developed to satisfy a requirement for such fasteners inaircraft structural locations having limited access for installation offasteners. However, such blind fasteners have been assigned relativelylow static strength allowables and are notoriously unreliable undervibration conditions and sustained cyclic flight loadings.

Recent emphasis on an increased fatigue life for airframes has led tothe use of nut and bolt installations wherein the shank of the boltcreates an interference fit with the hole to prestress the surroundingmaterial to reduce the mean overall stress level for the prevention offatigue cracks. Such interference fit installations have generally beenlimited to threaded fastening systems wherein access may be had to bothsides of the joint. Two-piece expandable sleeve assemblies, such asshown in US. Pat. 3,271,058 to Anderson, can be used to obtain somedegree of preload around the hole but require access from both sides ofthe joint for installation and, as is pointed out in the Anderson'patent, require a threaded bolt-to-nut connection or other externalholding means because of their tendency to loosen under vibrationloadings.

It is an object of this invention to provide a blind fastener systemwhich may be completely installed from one side of the joint, whichexpands to prestress the material surrounding the hole for increasedfatigue life, and which has static and vibration strength allowablesapproaching those of a conventional high strength threaded bolt and nutcombination.

A related object of this invention is to provide a twopart flush headblind fastener assembly which utilizes one-way serration notches actingagainst a smooth conical surface to provide an adjustable grip lengthand a locking interengagement of the two parts of the fastener systemwithin the hole.

A further related object of this invention is to provide, in a fastenerof the class described serration notches acting ahead of a break-offgroove to react a portion of the load required for installing thefastener to reduce the load acting on the break-off groove duringinstallation.

Other and further objects and advantages of this invention will becomeapparent in the discussion and description which follows.

SUMMARY The objectives of this invention have been achieved by atwo-part radially expanding double shank fastener assembly which in thepreferred embodiment comprises a flush head sleeve member with a smoothinner wall tapered at 0.020 inch per inch, and a matching conical drawpin member of correspondingly larger diameters by approximately 0.005inches. The pin member has a serrated surface which lockinglyinterengages with the smooth inner wall of the sleeve upon finalinstallation. A break-off groove is located on the pin along theserrated surface so that several lead-in serrations precede the grooveto reduce the installation tensile load acting on the groove to minimizepremature break-off problems of the prior art. When the fastenerassembly is initially inserted in a hole from the flush side of thejoint, the protrusion. on the opposite side of the joint is of the orderof the diameter of the fastener, a significant improvement over priorart blind fasteners which have a protrusion approximating the griplength of the fastener. When installed in a close tolerance hole, theassembly will radially expand to create a predetermined residual hooptension stress in the material surrounding the hole to improve thefatigue life of the joint.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a bolt assembly of thisinventionafter insertion from the flush side of the joint and prior tothe application of installation forces on the draw pin.

FIG. 2 shows the bolt assembly of FIG. 1 completely installed in thehole after application of installation DESCRIPTION OF PREFERREDEMBODIMENTS FIG. 1 is a cross-sectional view showing a blind boltassembly of this invention wherein an external sleeve member 6 and aninternal pin member 8 have been inserted into a close tolerance holepassing through structural members 10 and 12, from the upper or flushside of the joint. External sleeve member 6 comprises an annularextending end portion 14, a flush head-end portion 16 having an includedcone angle ranging from 60 to and preferably 70, and a central annularshank portion 18. The radius R is preferably greater than 0.03 inches asan enlarged radius has been found to assist in the smooth deformation ofmaterial around the hole upon expansion of the sleeve member. The sleeve6 is provided with a smooth inner wall 19 having a taper ofapproximately 0.020 inch per inch along its entire length.

The internal pin member 8 is preferably constructed of a materialsomewhat harder than that of sleeve 6, and includes a draw portion 30provided with a series of grip rings 31, for engagement with a draw pinpulling device of a type known in the art. A protruding headend 32 has aswaging surface 36, preferably disposed at an included cone angle ofless than and integrally connected to a conical shank portion 34 havinga surface with series of serration notches 35 formed therein.

The conical shank portion 34 has a taper'to match that larger diametersby approximately 0.005 inches. A break-off groove 38 is designed tofracture at a predetermined tensile load sufficient to radially expandand swage the sleeve into its final installed position. A pair ofserration notches 40 are located ahead of the break-off groove and serveto take a portion of the load which would otherwise pass throughgroove38 during initial relative movement of the pin 8 in expanding the sleeve.6. This expedient has been found to minimize premature break-offproblems.

From FIG. 1 it can be noted that the total initial protrusion of thefastener assembly is approximately the same as the diameter of thefastener, an improvement over many blind fasteners requiringsubstantially more protrusion clearance. Using the taper and geometriesdiscussed above the total travel of thepin 8 need be no more thanapproximately one-fourth inch to achieve the desired degree of expansionof the sleeve member 6. The total travel distance involved here has beendeliberately shortened to eliminate bending, sleeve stretching andgalling problems associated with certain prior art blind fasteners.

FIG. 2 shows the completed installation after application of pullingforces to the draw pin member 8 which expand the sleeve by contactbetween-the enlarged serration surface of the pin and the smooth innerwall 19' of the sleeve member 6, and swage the extending annular portion14 of the sleeve through contact with swaging surface 36 of the pin. Inthe development of this invention it was found that the one-way lockingserrations acting against a matching undersized smooth conical surfaceplus the swage action on the blind side of the joint will allow theassembly to be installed with relatively low-level pulling forces whileassuring a positive interlock to secure the joint under vibration loads,and static structural loads approaching those normally associated withhigh strength threaded fasteners. When fully installed as shown in FIG.2, a desired degree of radial expansion, such as the 0.005 inchesdiscussed earlier, will have been achieved and the material surroundingthe holewill have been prestressed with tensile hoop-tension loads whichwill substantially improve the fatigue life of the joint.

FIG. 3. shows the bolt of thisinvention installed in a honeycombsandwich panel, in which its economic and improved strength advantageshave been found to be significant over prior art fastening systems, manyof which require separate" load transfer spools embedded in the panelprior installation of a fastener. The fastener of this invention expandsin the manner illustrated to grip each of the panel face sheets and givevery good strength, vibration and fatigue properties without thenecessity of elaborate pretreatment of the hole structure. The sandwichpanel honeycomb core 50 is attached to face sheets 52 and 54 and joinedto structural member 56 by a fastener assembly of the type shown inFIGS. 1 and 2. As the draw pin 8 is pulled into the installed positionshown, the central shankportion 18 of external sleeve member 6expandsradially into the soft edged core webs while being restrained bythe relatively rigid face sheetsSZ and 54. This action, together withthe swaging of end portion-l4, serves to trap and hold the face sheetsfor load transfer purposes as well as to prestress the face sheetmaterial around the hole to prevent fatigue cracks. Therefore, theinstallation offers a structurally sound joint which requires only thedrilling of a close tolerance hole through the sandwich panel, andeliminates elaborate hole pretreatment, as, for example, the placementof a spool surrounded by potting compound in contact with the web andface sheets. I

FIG. 4 presents a plot of fatigue data obtained using conventional priorart blind fasteners of a type in widespread use today, and a plot ofsimilar data ob-- tained with the fastener herein disclosed. The testswere performed on identical standard dog-bone noload transfer typetensile specimens. The ordinate represents tensile stress levels on thenet specimen area surrounding the hole. The abscissa is in terms ofcycles to failure plotted on a logrithmic scale. The improvement shownby these plots is striking. For example, note that at the 50,000 psistress level, in the range criticalfor titanium airframe structures, theimprovement is from approximately 120,000 cycles to over 1,000,000cycles. Accordingly, thedisclosed fastener system represents asignificant advance in the aircraft fastener art in terms of fatiguestrength improvement as well as the advantages previously discussed.

The invention has been described and illustrated in detail usingpreferred dimensioning and embodiments developed to date. While most ofthe specimens tested have been designed to obtain a diametricalinterference of 0.005 inches for preload purposes, the basic conceptshave been demonstrated to be applicable to interferences ranging from0.001 to 0.012 inches, the optimum amount of interference to be selectedas a function of the specific joint parameters under consideration, suchas material properties, fastener size, load transfer ratio and jointconfiguration. Variations and modifications will occur to personsskilled in this art without departing from the spirit and scope of theinvention. Accordingly, it is intended in the appended claims to coverall suchvariations and modifications.

What is claimed and desired to be secured by US. Letters Patent is:

1. A blind fastener system which may be installed from one side of astructural joint comprising in combination: an external sleeve and aninternal pin arranged to lockingly interengage within a hole passingthrough said joint; wherein said sleeve is provided with a conicalsmooth inner wall tapering uniformly from. a minimum diameter at the endof the sleeve nearest the side of the joint from which the fastener isto be in-. stalled, to a maximum diameter at the other end of saidsleeve; and, wherein said internal pin is provided with a conicallyshaped shank tapered to match the taper of said inner wall, said shankincluding a serrated section having a plurality of surface notches forengagement with the smooth inner wall of said sleeve, the diameters ofsaid shank being greater thanthe corresponding diameters of said innerwall by an amount such that when said fastener system is fully installedin said joint, said pin member will force said external member to expandradially outwardly to a prestressed condition and said serrated sectionsurface notches will lockingly engage the smooth wall of said sleeve toprevent disassembly of said fastener system; wherein said internal pinis provided with a protruding head-end portion having a conical swagingsurface with an included cone angle of less than 90 adapted to engageand swage outwardly an end portion of said sleeve, and said pin is alsoprovided with a break-off groove of substantially reducedcross-sectional area such that at a predetermined installation loadsufficient to fully install said fastener, said break-off groove willfracture to disengage the means used to install the fastener; whereinsaid pin is provided with at least one serration notch ahead of saidbreak-off groove to reduce the initial installation forces passingthrough said break-off groove region; wherein said smooth inner wall istapered substantially .020 inch per inch along its length; wherein saidsleeve has a flush conical head with an included cone angle ranging from60 to adapted to seat in a countersink in the face of the joint on theside from which the fastener is installed; wherein said sleeve has aradiused surface having a minimum radius of curvature of 0.03 inchescontiguous with the surface of said conical head; and wherein when saidfastener system is fully installed in said hole, the exterior shape ofsaid sleeve has been diametrically expanded to cause an interference fitwith the walls of said hole ranging from 0.001 to 0.012 inches togenerate hoop tension stresses in the surrounding material for thepurpose of increasing the effective fatigue life of the joint.

1. A blind fastener system which may be installed from one side of astructural joint comprising in combination: an external sleeve and aninternal pin arranged to lockingly interengage within a hole passingthrough said joint; wherein said sleeve is provided with a conicalsmooth inner wall tapering uniformly from a minimum diameter at the endof the sleeve nearest the side of the joint from which the fastener isto be installed, to a maximum diameter at the other end of said sleeve;and, wherein said internal pin is provided with a conically shaped shanktapered to match the taper of said inner wall, said shank including aserrated section having a plurality of surface notches for engagementwith the smooth inner wall of said sleeve, the diameters of said shankbeing greater than the corresponding diameters of said inner wall by anamount such that when said fastener system is fully installed in saidjoint, said pin member will force said external member to expandradially outwardly to a prestressed condition and said serrated sectionsurface notches will lockingly engage the smooth wall of said sleeve toprevent disassembly of said fastener system; wherein said internal pinis provided with a protruding head-end portion having a conical swagingsurface with an included cone angle of less than 90* adapted to engageand swage outwardly an end portion of said sleeve, and said pin is alsoprovided with a break-off groove of substantially reducedcross-sectional area such that at a predetermined installation loadsufficient to fully install said fastener, said break-off groove willfracture to disengage the means used to install the fastener; whereinsaid pin is provided with at least one serration notch ahead of saidbreak-off groove to reduce the initial installation forces passingthrough said break-off groove region; wherein said smooth inner wall istapered substantially .020 inch per inch along its length; wherein saidSleeve has a flush conical head with an included cone angle ranging from60* to 85* adapted to seat in a countersink in the face of the joint onthe side from which the fastener is installed; wherein said sleeve has aradiused surface having a minimum radius of curvature of 0.03 inchescontiguous with the surface of said conical head; and wherein when saidfastener system is fully installed in said hole, the exterior shape ofsaid sleeve has been diametrically expanded to cause an interference fitwith the walls of said hole ranging from 0.001 to 0.012 inches togenerate hoop tension stresses in the surrounding material for thepurpose of increasing the effective fatigue life of the joint.